A further increase in demand for an electrochemical device such as a lithium ion secondary battery which is compact and lightweight, has high energy density, and is capable of repeatedly charging and discharging is expected even in view of environmental concerns. The lithium ion secondary battery has high energy density and thus is used in the fields of mobile phones, laptop personal computers, and the like. In addition, with an increase or development in use application, the need for further improvement in performance, such as a reduced resistivity or an increased capacity, in the electrochemical device has increased.
For example, in Patent Literature 1, a mixture of two styrene-butadiene copolymer latexes different in amount of bound styrene is used as a binder for the purpose of improving the mechanical strength and battery characteristics of a negative electrode in lithium ion secondary batteries having a high discharge capacity.